Impact of Water Saturation on Gas Storage in Clay-rich Shale
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AbstractFor shale gas reservoirs, to evaluate the total gas-in-place (GIP) requires us to estimate the adsorbed gas amount accurately because the adsorbed gas generally constitutes 40%-85% of the total GIPs. However, under geologic conditions, water saturations always exist in shale reservoirs, which largely decreases the natural gas adsorption capability of shale, thus affecting the reliability of shale gas resources evaluation. In this thesis, we have built a mathematical model for calculations about gas adsorption in shale clay in different water environments, where the Langmuir adsorption equation is adopted for the gas-liquid interaction between methane and dry clay surface, and the Gibbs equation is used to characterize the gas-liquid interaction between methane and an adsorbed water film. Through our calculation, with the water saturation in a 4-nm slit pore being 20%, methane adsorption capacity is decreased by 55.4%, compared with dry conditions; when the water saturation in a 4-nm capillary is 36%, methane adsorption capacity is decreased by 80%. This proposed model is then extended to porous media which follows the log-normal pore size distribution (PSD). Based on the results, we have found that the shapes and sizes of nanopores in shale can affect the moisture distribution features and gas adsorption capability. Influences of water on methane adsorption capability mainly acts as (i) an adsorbed water film in large pores decreases the affinity between methane and pore-walls; (ii) capillary water totally blocks some small pores and makes them loose abilities to adsorb methane. When the water saturation of the slit-pore porous media reaches 43%, with the 2-nm slit pores blocked by water, the gas adsorption capability declines by 50% compared with dry conditions. When the water saturation of the capillary porous media arrives at 45%, with the 3-nm slit pores blocked by moisture, the methane adsorption capacity is deceased by 70% compared with dry conditions.
CitationZhu, L. (2020). Impact of Water Saturation on Gas Storage in Clay-rich Shale (Unpublished master's thesis). University of Calgary, Calgary, AB.
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